1. Field of the Invention
The present invention is generally related to semiconductor processing systems and, more particularly, to temperature control of a substrate using rough contact or micron-size gaps in a substrate holder.
2. Discussion of the Background
Many processes (e.g., chemical, plasma-induced, etching and deposition) depend significantly on the instantaneous temperature of a substrate (also referred to as a wafer). Thus, the capability to control the temperature of a substrate is an essential characteristic of a semiconductor processing system. Moreover, fast application (in some important cases, periodically) of various processes requiring different temperatures within the same vacuum chamber requires the capability of rapid change and control of the substrate temperature. One method of controlling the temperature of the substrate is by heating or cooling a substrate holder (also referred to as a chuck). Methods to accomplish faster heating or cooling of the substrate holder have been proposed and applied before, but none of the existing methods provide rapid enough temperature control to satisfy the growing requirements of the industry.
For example, flowing liquid through channels in the chuck is one method for cooling substrates in existing systems. However, temperature of the liquid is controlled by a chiller, which is usually located at a remote location from the chuck assembly, partially because of its noise and size. However, the chiller unit is often very expensive and is limited in its capabilities for rapid temperature change due to the significant volume of the cooling liquid and to limitations on heating and cooling power provided by the chiller. Moreover, there is an additional time delay for the chuck to reach a desired temperature setting, depending mostly on the size and thermal conductivity of the chuck block. These factors limit how rapidly the substrate can be cooled to a desired temperature.
Other methods have also been proposed and used, including the use of an electric heater embedded in a substrate holder to affect heating of the substrate. The embedded heater increases the temperature of the substrate holder, but the cooling thereof is still dependent on cooling liquid controlled by a chiller. Also, the amount of power that can be applied to the embedded heater is limited, as the chuck materials in direct contact with the embedded heater may be permanently damaged. The temperature uniformity on an upper surface of the substrate holder is also an essential factor and further limits the rate of heating. All of these factors place limits on how rapidly a temperature change of a substrate can be accomplished.